Cerium Levels in Coarse and Fine Airborne Particulate Matter in El Paso, Texas, U.S.A.

In Europe, ceria or cerium dioxide (CeO2), introduced into diesel road fuel as nanoparticles, has significantly reduced emissions as well as increased vehicle mileage. Concerns have been raised there about the fate and potential health risks associated with the nano-ceria (n-Ce) emitted in diesel exhaust. In the U.S.A., on-road use of n-Ce additives is still under regulatory study. Because of possible future use, it is important to establish baseline data on current levels of airborne cerium. This is of special interest in El Paso, Texas, U.S.A., which shares a common air shed with contiguous Ciudad Juarez, Chihuahua, Mexico where n-Ce might also be used. This study analyzed weekly total concentrations of cerium in El Paso air at 8 sampling stations during 2006 to 2009. A PM10 dichotomous sampler simultaneously collected PMC (the PM10 to PM2.5 fraction) and PMF or PM2.5. An X-ray fluorescence instrument measured cerium in the two PM fractions. Results indicate higher levels of airborne cerium in PMC (~2 ng/m3 average for all sites for entire study period) than in PMF (~1 ng/m3). Higher values for coarse and to a lesser degree for fine PM are associated with sites proximal to the urban core of the binational El Paso—Cd. Juarez metroplex. This indicates significant anthropogenic contribution to airborne cerium in El Paso; abrasion of vehicle parts that incorporate cerium-doped alloys, residual cerium catalysts from gasoline refining, other commercial and industrial debris, and open-air waste burning are likely sources. No overall seasonal patterns were evident other than a possible decrease in PMC during the summers; this is consistent with a significant anthropogenic contribution. Background Ce levels are low relative to those anticipated from possible future use of n-Ce additives, and thus any such increases would be readily detectable in future monitoring. Current Ce levels appear to pose no local health risk relative to tentative U.S. Environmental Protection Agency reference concentrations.